There is a great demand for large cast shapes and large cast weld structures in petrochemical, heat treating, and other fields. The desired metallic properties are long life, high hot strength, good resistance to thermal fatigue and thermal shock and good weldability at moderate cost.
The nickel-base superalloys developed primarily for aircraft jet engine components since World War II achieve hot strength through solid solution hardening by inclusion in the alloys of large quantities of molybdenum and, to a lesser extent, niobium (columbium), plus precipitation at or along the grain boundaries of very fine particles of gamma prime, a compound formed between nickel and fairly large amounts of aluminum plus titanium. Since aluminum and titanium are readily oxidized in air melting practices, these nickel-base superalloys are melted and poured in a vacuum or an inert atmosphere. This requirement plus the high cost of such alloys make them impractical for the present application.
Cobalt-base superalloys were also developed for aircraft jet engine components. These alloys derive their hot strength primarily by solid solution hardening by elements of the group Mo, W, Nb, and Ta, plus precipitation of refractory carbides along the metallic grain boundaries. A few cobalt-base alloys and cobalt-nickel base alloys have also employed gamma prime hardening. Cobalt remains a relatively scarce and expensive metal, and therefore cobalt-base superalloys are far too expensive for the large structures discussed here.
Both cast and wrought iron-nickel-chromium-base alloys have been the economical choice in these applications. Wrought alloys of less than about 0.15% C content, sometimes containing about 1.5% or less combined content of aluminum plus titanium, have been employed as headers, manifolds, cones, and transfer lines. These alloys have excellent weldability and good resistance to thermal cracking in service. They have relatively very low hot strengths.
However, the higher hot strength, higher carbon casting alloys permit reduction in wall thickness, reduction of metal weight and better rate of heat transfer. Nickel-iron-chromium-base alloys have been modified to include up to about 1% Nb, up to about 5% W and up to about 15% Co for increased hot strength. In general those cast alloys of about 0.45 to 0.55% C contents have higher hot strengths but very poor to almost no weldability, while cast alloys of about 0.35 to 0.40% C contents have substantially lower hot strengths but some degree of weldability.
These carbon strengthened alloys tend to age and embrittle in service. Thus, they may easily crack during thermal cycling. Alloys that, on a creep-rupture test basis, should last about ten years have sometimes been found to fail in cracking after perhaps one year or so in service. Also, in some applications it is desirable to weld repair some components after periods of service.
Alloys which include substantial amounts of iron in their formulation are, in general, considerably less expensive than nickel-base, iron-free alloys for at least two reasons. They may employ much lower cost ferroalloys to make up their contents of chromium, and sometimes other components, as contrasted to the higher cost pure chromium and other metals required in nickel-base alloys, In addition, the mere replacement of even a portion of the moderately expensive nickel by very low cost iron represents substantial component cost savings. In alloys of the present invention a third extremely important advantage of including substantial quantities of iron in their formulation is that they develop higher hot strengths than the far costlier iron free nickel base alloys of the equivalent hardness and weldability and of the same tungsten contents.
Thus, there has remained a great demand in oil refineries, petrochemical plants, heat treating equipment, and other applications or moderate cost, for air meltable nickel-iron-chromium-base alloys that do not require large amounts of carbon in their formulation for hot strength and that have exceptional weldability as cast. It is further desirable in some instances that such alloys retain good room temperature ductility and weldability after periods of service at high temperature.
English, et al, U.S. Pat. No. 2,540,107, disclosed a modification of alloy type HP containing 40 to 60% Ni, 22 to 34% Cr and 4 to 6.5% W and known commercially as alloy 22H. English, U.S. Pat. No. 3,607,250, disclosed an improved version known as super 22H, containing 40 to 55% Ni, 27 to 33% Cr, 4 to 5% W and 2 to 4.5% Co. Avery, U.S. Pat. No. 3,127,265, disclosed an alloy know as Supertherm, which contains 26 to 42% Ni, 22 to 32% Cr, 3 to 16% W, 9 to 26% Co, 0.3 to 0.95% C, 0.5 to 2% Si, and the balance, if any, iron. In practice, this alloy nominally contains about 35% Ni, 26% Cr, 15% Co, 5% W, 0.5% C, 0.7 % Mn, 1.6% Si and 21% Fe. British Pat. No. 1,046,603 disclosed a nickel base alloy known as MO-RE 2, containing 26 to 38% Cr, 10 to 25% W, less than 1% C, less than 0.2% each of Mn and Si, and the balance Ni. All four of these alloys are characterized by low cold ductility and little if any weldability.
Nickel base superalloys, other than MO-RE 2, and cobalt base superalloys have employed up to 15% W, up to 14.5% Mo, up to 5.6% Nb and up to 9% Ta in order to attain high substitutional solid solution matrix hardening and strengthening, and in many cases, to additionally form hard refractory carbides. There is some information in the literature concerning the solid solubility limits of these four elements at various elevated temperatures and as affected by different levels of chromium in nickel base alloys, but there is almost nothing reported concerning how much these solubility limits are affected by various levels of carbon and iron additions to complex alloys which otherwise contain various levels of nickel, chromium, manganese, silicon, aluminum, titanium and possibly cobalt.